The invention relates to muting signal generation circuits for an FM receiver and more particularly to a muting signal generation circuit for generating a muting signal during periods of detuning or when an S/N ratio is degraded.
In a receiver, a muting circuit is operated to thereby mute a line output during times when detuning occurs, for example, during selection of broadcasting stations or when the S/N ratio is degraded such as may be caused by, for example, mutual modulation interference, multi-pass interference or adjacent channel interference.
In FIG. 1, there is shown a block diagram of a prior art muting control section of an FM receiver in which the output signal V.sub.O, which varies in accordance with the frequency changes in a received signal as shown in FIG. 3, of an FM detector 1 is applied to both a low-pass filter 2 and a high-pass filter 3. The low-pass filter 2 passes only the d.c. component of the detected output signal V.sub.O. With this arrangement, the output of the low-pass filter 2 increases in proportion to the d.c. component of the detected output signal V.sub.O and hence with undesirable deviations in the frequency f to which the receiver is tuned. The output of the low-pass filter 2 is applied to the next stage, the detuning detector 4. When the frequency to which the receiver is tuned is above or below a predetermined value relative to a central frequency f.sub.0 of a received signal from a broadcasting station, the output signal from the detuning detector correspondingly shifts above or below a reference level V.sub.ref. The detuning detector 4 produces, for example, a high level signal when the shift of the detected output signal V.sub.O relative to the reference voltage V.sub.ref is equal to or above a predetermined value. The output signal from the detuning detector 4 operates a muting signal generator 5 which generates a muting signal when the output signal from the detuning detector is at the high level for a predetermined period of time. The muting control circuit 6 is actuated in response to the muting signal.
On the other hand, only high-frequency components which are produced when the S/N ratio is degraded are passed by the high-pass filter 3 from the FM detection output signal V.sub.O. The high-frequency noise passed therethrough is then subjected to detection and smoothing in a noise detector 7 which receives its input from the high-pass filter 3. The muting signal generator 5 is operated in accordance with the output of the noise detector 7.
The above-described prior art arrangement is disadvantageous in that the circuit construction is relatively complicated because both a detuning detector 4 and a high-frequency noise detector 7 are required. In addition, when the circuit is constructed on an IC chip, it is further disadvantageous in that two pins are required for connecting the respective outputs of the low-pass filter 2 and the high-pass filter 3 to the IC circuit.